Enhancement - Supports '--' on the command line

- Using '--' on the command line separates the left hand side, where
  task is free to interpret arguments in the usual way, and the right
  hand side, which is then assumed to be part of the task description,
  and is not interpreted by task.

src/task.cpp

@@ -272,6 +272,8 @@ static std::string longUsage (Config& conf)
       << "  task add \"quoted ' quote\""                                      << "\n"
       << "  task add escaped \\' quote"                                       << "\n"
       <<                                                                         "\n"
+      << "The argument -- tells task to treat all other args as description." << "\n"
+      <<                                                                         "\n"
       << "Many characters have special meaning to the shell, including:"      << "\n"
       << "  $ ! ' \" ( ) ; \\ ` * ? { } [ ] < > | & % # ~"                    << "\n"
       << std::endl;